Analogue to digital conversion is an integral part of a number of technologies, in particular radio communication technologies. Radio communication schemes transmit data via a transport medium, such as the air, in the form of analogue signals. Such analogue signals typically comprise a radio-frequency (RF) frequency carrier component modulated by one or more lower frequency data components. Modern radio communication equipment utilises digital signal processing to operate on at least the data components of received signals. Hence, analogue to digital conversion is required before the data components of received signals can be processed.
Generally, analogue to digital converters sample an analogue input signal periodically and produce a digital representation of the magnitude of the analogue input signal at the time of the sample. The accuracy of the digital representation depends on the number of quantisation levels the analogue to digital converter has. The digital representation is typically generated on the basis of the quantisation level that is closest to the magnitude of the analogue input signal at the time of the sample. The range of different quantisation levels provided is termed the operating range of the analogue to digital converter. The spacing of the quantisation levels defines the resolution of the analogue to digital converter.
In order to accurately represent an analogue signal as a digital signal, the sampling rate of the analogue to digital converter should sample the analogue signal at a rate equivalent to at least half the frequency of the highest frequency of interest in the analogue signal. If this criterion (known as the Nyquist criterion) is not met, then aliasing of the signal may occur during the conversion. The maximum sampling rate of an analogue to digital converter is most commonly limited by the throughput of the analogue to digital converter. Hence, many conventional receiver arrangements convert the data components of received signals to a lower frequency before performing analogue to digital conversion. A receiver arrangement (or operation, process, apparatus, method, etc.) that does not involve such a conversion to a lower frequency may be referred to as a direct radio-frequency to digital conversion receiver arrangement.
Frequency down-conversion is typically achieved by mixing the signal with a local oscillator signal operating near the carrier frequency of the received RF signal. The down-converted signal is then filtered, typically using a low pass filter, to remove signal components outside the desired frequency range. Frequency down-conversion may convert the signals of interest directly to baseband or low frequency, or may alternatively use multiple down-conversion stages and one or more intermediate frequencies. The result of this frequency down-conversion is to move the frequency of the signals of interest to within the operating frequency range of the given analogue to digital converter arrangement.
More recently, direct RF to digital converters have been used that convert a received RF signal to a digital signal without first performing down-conversion on the received signal. Due to the high sampling rate required of an analogue to digital converter used as a direct RF to digital converter, suitable architectures often have undesirable characteristics, such as small number of quantisation levels, high cost/semiconductor area, high power consumption, etc.
A known analogue to digital converter arrangement capable of operating at suitably high sampling rates is known in the art as a flash converter. A flash converter utilises a number of comparators, each arranged to compare an analogue input signal with a different reference voltage. The various reference voltages are distributed over the operating voltage range of the flash converter. The comparators are arranged in parallel such that each consecutive comparator utilises an increasing reference voltage. The result of this arrangement is that the output of the comparators is thermometer coded. An encoder is then used to convert the thermometer coded comparator output into a binary coded signal using priority encoding logic.
Due to the priority encoding logic required and the necessary throughput, encoder hardware scales poorly when an increasing number of comparators is used. As a result, flash converters with high resolutions are rarely used at high sampling rates. Further, the output of a flash converter is clocked at the sampling sate of the flash converter, which, at radio-frequencies, is typically too fast to be processed by conventional communication equipments.
Hence, it would be desirable to provide improved measures for performing direct RF to digital conversion.